Neil, do you know what MMC did as the cure ?

Yes - they listed:

Machine Shop Supply Fuel Adapter.

Tank Rubber Gasket

Petrol Hose 1/2 Inch Bore

Petrol Cap Side Vent 12mm

I assume bigger vents/bores and an especially made up 'supply fuel adapter' - but I don't know what this is.

It was worth having done as filling the car was beyond tedious. It was painfully slow (gingerly pressing the trigger etc) but also not really knowing if the car was actually full.

Worth adding that previously I would end up with my feet covered in petrol as it would also suddenly woosh out of the filler cap. This has stopped too.

Many thanks

Does yours have this mis filling plate removed now ?

I ask because I have a pal with a trad that has problems of fuel burp & can never get a full tank, & I suspect the unleaded only breather plate below could be removed

The inside of the cap looks as it did before - so I don't think that plate has been removed.

I cant easily fully fill the tank. So this is interesting.

I had this problem with my series 1 roadster when new. I found that the plate with all the holes in pictured above rotates very freely. Only when the hinge is at 12 o'clock will the car fill properly.
That's my memory of it, I just rotated the plate using a pencil and the problem went away.
I should call it the flap, not the plate.
Nick

Just tried the 12 oclock apporach & that seems to help

I have to say that it's a bit of a game changer being able to fuel easily - but also recognise that it's major faff to take it to the factory. I am glad I have had mine done.

That was one of the things that used to drive me a little bit (more) nuts - it was a big change as you say.